Nature - USA (2020-01-23)

494 | Nature | Vol 577 | 23 January 2020

Article

High-resolution elemental mapping images (Fig. 3f) reveal that Y spe-
cies are mainly monoatomically dispersed into the layers, associated
with a few Y clusters, possibly accounting for the many Y–S bonds pre-
sent (Supplementary Fig. 46). A typical 2H structure is visualized in an
aberration-corrected TEM image (Fig. 3c), consistent with X-ray diffrac-
tion (Supplementary Fig. 38) and Raman analysis (Fig. 3h), indicating
that the heteroatoms remain stable within the host 2D transition-metal
chalcogenide structure. To further verify the Y–S bonds in Y-doped
WS 2 , we conducted X-ray absorption near-edge fine structure (XANES)
spectroscopy measurements. In the case of Y-doped WS 2 , the absorp-
tion edge in Y K-edge XANES spectra (Fig. 3i) is close to that of yttrium
sulfide, suggesting that the Y is in the sulfide state. The Fourier trans-
form spectra resulting from the analysis of Y-doped WS 2 by extended
X-ray absorption fine structure (EXAFS) spectroscopy (Fig. 3j) show
a dominant peak at 2.2 Å, indexed to the Y–S bonds in comparison
with yttrium sulfide. Theoretically, the energy of 2H WS 2 is much lower
than its 1T phase (>0.6 eV per formula unit). With increasing Y doping
levels, the energy differences between 2H and 1T phases noticeably
decrease, but it remains difficult to reverse the relative stability (Sup-
plementary Fig. 68). Owing to the substitution of W (valency +4) in
WS 2 by low-valence Y (+3), the charge densities near the Fermi level
tend to be localized around Y atoms, thus going against electron

transfer (Supplementary Fig. 69). Moreover, the increase of defect

scattering by Y dopants would also reduce the electrical conductivity.

Thus, the electrical conductivity of Y-doped WS 2 is experimentally

measured to be 2.31 × 10−3 S cm−1, much lower than that of pure WS 2

(8.13 × 10−2 S cm−1) (Supplementary Table 3). This is in contrast to those

reported for d orbital electron-enriched metal (Re, Nb)-doped MoS 231 ,^32

and our Nb-doped TiSe 2 (Supplementary Figs. 47–50), exhibiting sub-

stantial improvements in the electrical conductivities (Supplementary

Figs. 51, 70 and Supplementary Table 3).

2D heteroatoms (Y, P) co-doped WS 2 (1T)

While engineering the quaternary MAX phase (W2/3Y1/3) 2 AlC, other

vapours such as phosphorus could be easily and simultaneously

introduced into the synthetic system, creating both Y and P co-doped

WS 2 (Supplementary Figs. 52–56). As illustrated by the Raman spec-

tra (Fig. 4d), for Y, P co-doped WS 2 there are three dominant peaks

at 130 cm−1 (J 1 ), 258 cm−1 (J 2 ) and 406 cm−1 (J 3 ), corresponding to the

vibration modes of the 1T phase^6 ,^33 , as well as two peaks at 348 cm−1

and 414 cm−1, indexed to the E2g^1 and A1g peaks of the 2H phase, respec-

tively. The presence of the 1T phase in real space can be visualized

via aberration-corrected STEM images (Fig. 4a and Supplementary

ab c

de

f gh

TMD-MoS

2

Intensity (a.u.)

TMD-MoS 2

MAX-Mo 2 GeC

2 T (º)

10 20 30 40 50 60 70 80

1 μm

(002)

(004)

(100)

(102)(103)

(006)(105)

(110)(008)

(107)(203)

MAX-Mo

GeC 2

H 2 S

100 nm 2 nm

Hexagonal MoS 2

d (100) = 0.28 nm

1 μm 200 nm 1 nm

TMD-TiSe

2

Fig. 2 | Structural characterization of 2D transition-metal chalcogenides
derived from MAX phases. a, Schematic illustration of the conversion of MAX-
Mo 2 GeC to accordion-like MoS 2 under H 2 S gas at 1,073 K. b, X-ray diffraction
patterns of MAX-Mo 2 GeC and accordion-like MoS 2. c–e, Scanning electron
microscope (c), sectional TEM (d) and high-resolution TEM (e) images of

accordion-like MoS 2. f, Crystalline structure of 1T TiSe 2. g, Scanning electron

microscope image and sectional TEM image (inset) of accordion-like TiSe 2 ,

indicating its highly expanded structure. h, Atomic-resolution STEM image of

TiSe 2 layers and the corresponding atomic configuration (inset) of the 1T

phase. Purple and green balls represent Ti and Se atoms, respectively.